Hereinafter, a comb-line filter is used as an example to describe a resonator filter coupled with conductive plates that have curved surfaces according to the present invention. The present invention is well-adapted to be used as a filter such as a comb-line filter or as an interdigital filter. However, it is understood that the present invention is not limited to be used only as a comb-line filter.
In order to efficiently use the available frequency spectrum in microwave communications systems, a corresponding frequency band must be divided into a plurality of narrow frequency bands. That is, channelization and multiplexing are required. Therefore, it is essential to develop a filter that satisfies stringent performance criteria for achieving channelization and multiplexing.
As a result, a narrow band filter must have a high frequency selectivity to efficiently use the available frequency resource. Therefore, narrow band filters are often configured as high order filters. Narrow band filters must also function by eliminating any unwanted adjacent harmonic frequency band signal except for the selected signal in pass-band frequencies. Since comb-line filters satisfy these above mentioned characteristics of the narrow band filter, comb-line filters are widely and commonly used as key constituents in wireless communication systems. The comb-line filter according to the related art will be described with reference to FIG. 1 hereinafter.
FIG. 1 is a perspective view of a comb-line filter having resonators connected to an input/output connector in accordance with the conventional art.
As shown in FIG. 1, the conventional comb-line filter includes an input/output coaxial connector 12 for inputting or outputting a signal; a plurality of resonators 13 for resonating; and a housing 11 for housing the comb-line filter.
The conventional comb-line filter generally includes the conductive resonators 13 made of conductors having good electrical conductivity characteristics and are connected together in a side by side manner. Because the comb-line filter is smaller than other types of filters having transmission lines, it is possible to avoid un-wanted harmonic signals that are adjacent to the harmonic frequency band. Generally, comb-line filters include resonators 13, shaped like a rod, in the housing 11 and includes input/output unit 12 which is made of a coaxial connector. The input unit 12 is connected to the first resonator 13 and the output unit 12 is connected to the last resonator 13. The resonators 13 are electrically coupled to one another through an iris (inductive coupling) or through a probe (capacitive coupling).
Although the comb-line filter was introduced during the 1960s, there still exist interest in developing comb-line filters that exhibit superior characteristics to those of more conventional comb-line filters. A considerable amount of research has been focused on reducing the volume of the filter, controlling a coupling amount and controlling a center frequency band.
For example, a first conventional technology was introduced in U.S. Pat. No. 4,761,624 entitled “Microwave band-pass filter.” The first conventional technology discloses a dielectric material used instead of an air layer in order to reduce a volume of the comb-line filter that controls the coupling amount by an air space between resonators as shown in FIG. 1. That is, rod shaped holes are formed at a dielectric box, and the hole are filled with dielectric material. The resonators are coupled through a thin conductive strip formed on the dielectric box.
Although the volume of the comb-line filter can be substantially reduced due to using the dielectric material, it is very difficult to finely tune the comb-line filter after manufacturing the filter due to the fixed size of the strip and the space. Accordingly, the adjustable coupling amount is limited by the size of the strip and the space. Such a limitation of designing the filter requires numerous trials and errors to develop a filter part satisfying a target specification. If stringent specifications are required, it is very difficult to manufacture the part to satisfy the specifications. Therefore, a functional part to modify or to control the coupling amount after the manufacturing the filter is often required.
Meanwhile, if irises are inserted between resonators in the filter, a narrow gap is generally required to obtain the requisite coupling amount between the resonators. That is, the filter with the iris generally has a shorter length than a filter without the iris. Therefore, the comb-line filter for the microwave band-pass filter is generally designed to have an iris between the resonators. However, this requires a fine manufacturing process since the size of the iris significantly influences the electrical performance. Also, another drawback is that the manufactured iris cannot be replaced.
In order to overcome these drawbacks, a second conventional technology was introduced in U.S. Pat. No. 6,664,872 entitled “Iris-less comb-line filter with capacitive coupling element. In the second conventional technology, a probe is used as the coupling between the resonators to correct the manufactured iris caused by a mechanical error or a designing error instead of using the iris.
Although the second conventional technology can realize a number of advantages in manufacturing the housing, it is difficult to replace the probe manually. Also, it is impossible to finely tune the second conventional device after manufacturing because the iris coupling uses a tuning screw to fine tune.
Research has continued to actively develop a comb-line filter for an intelligent wireless communication system as a potential next generation wireless communication system, in which it is hoped to be used for a communication system having a fixed center frequency and bandwidth.
The intelligent wireless communication system processes various frequency bands and communication schemes. Such an intelligent wireless communication system must have a function of communicating with other systems within a selected frequency. Therefore, the intelligent wireless communication system requires a filter that tunes a center frequency. In order to achieve this a third conventional technology was introduced in US Patent Publication Application No. 2003/02190109, entitled “Electrically tunable combline filters tuned by tunable dielectric capacitors.” In this third conventional technology, a variable dielectric capacitor is coupled to the resonators in the comb-line filter. The variable dielectric capacitor controls a capacitance value by controlling voltage. As a result, the resonance frequency is electrically controlled by changing the capacitance value through voltage, which was controlled using a tuning screw. However, the third conventional technology is distinguished from the present invention because the third conventional technology relates to changing of the resonance frequency and the present invention is related to changing of input/output coupling amount. That is, the third conventional technology allows the resonator to change a center frequency using the variable dielectric capacitor. In contrast, the present invention increases the coupling amount using a support member having curved surfaces similar to the resonator and controls the coupling amount by additionally forming a portion on the support member.
As described above, the first, the second and the third conventional technologies relate to a coupling between resonators, miniaturization and electric-control of resonance frequency in a conventional comb-line filter. However, the input unit and the output unit are directly connected to the resonators and the resonators are separated from one another using a probe supporting member according to the conventional art.
If the input unit and the output unit are directly connected to the resonators, it is difficult to accurately connect the output unit with the resonators. Moreover, the coupling part may be easily separated. Therefore, a careful, precise and accurate process is required.
If a support member is used to couple the probe and the resonator, very narrow spaces must be maintained to obtain maximal coupling because the diameter of the probe is very small. Therefore, it is very difficult to realize such a filter due to these requirements.
In particular, a filter must provide a center frequency and a bandwidth identical to a design value in order to satisfy the strict performance specifications. However, there are no methods that can effectively control a coupling amount of an input unit and an output unit. Until now, a dielectric or a conductive plate was used to control the coupling amount as a supplementary coupling device.
However, it is impossible to control the coupling amount of the input unit and the output unit of the conventional resonator filter made of conductive material, and the conventional resonator filter may be an unstable mechanical structure when the input unit and the output unit are coupled together. Therefore, the coupling of the input unit and the output unit must be improved to develop the resonator filter to satisfy the strict performance specifications. That is, there are great demands to develop a filter capable of controlling a coupling amount of an input unit and an output unit and that has a stable mechanical structure when the input unit and the output unit are coupled together.